Plugins: Failed to load 'Assets/Plugin/GUINativePlugin.dll' because one or more of its dependencies could not be loaded.

When opening the GUINativePlugin.dll in the free Dependency Walker tool (http://www.dependencywalker.com/), you can see that the .dll has a dependency on TEXTURECONVERTER.DLL that is not included in the project. And this is why DllNotFoundException is thrown, because we are unable to load the native GUINativePlugin.dll due to a missing dependency.